Discussion

Some Sea-Bird CTDs can be equipped with a Sea Tech or WET Labs Flash Lamp Fluorometer. For these fluorometers, the CTD configuration file requires the scale factor and offset to enable the software to calculate the concentration:

Concentration = (voltage * scale factor / 5) + offset

These fluorometers have internal, switch-selectable ranges. The scale factor is dependent on the fluorometer range:

Fluorometer

Range (milligams/cubic meter)

Scale Factor

Sea Tech

0 - 3

3

0 - 10 (factory-set default)

10

0 - 30

30

0 - 100

100

0 - 300

300

0 - 1000

1000

WET Labs FLF

0 - 100

100

0 - 300 (factory-set default)

300

0 - 1000

1000

The offset can be calculated by measuring the fluorometer voltage output when its light sensor is completely blocked from the strobe light by an opaque substance such as heavy black rubber:

offset = (-scale factor * output voltage ) / 5

Note: Scale factor and offset can be adjusted to fit a linear regression of fluorometer responses to known chlorophyll a concentrations.

Using Seasave V7 to Determine Offset

In Seasave V7, select Configure Inputs; the Configure Inputs dialog box appears. On the Instrument Configuration tab, click Open (open an existing .con file), Create (create a new .confile), or Modify (modify an existing .con file). Input / verify the scale factor for the fluorometer from the table above.

In the Display menu, select Add New Fixed Display Window. In the Fixed Display dialog box:

Double click on the voltage channel used by the fluorometer from the list of available voltage numbers (see CTD Configuration Sheet at beginning of CTD manual for voltage channel assignments as shipped from the factory).